Polymeric materials are processed using a variety of techniques. Many techniques employ an extruder which plasticates polymeric material by the rotation of a processing screw within a barrel. Some processing techniques, such as injection molding or blow molding, are discontinuous. That is, during operation, the screw does not plasticate polymeric material continuously. For example, the screw may stop rotating after a charge of polymeric material mixed with blowing agent is accumulated downstream of the screw, and thus cease to plasticate polymeric material, as well as during an injection molding cycle when the screw moves in a downstream direction to inject polymeric material into a mold.
Polymeric foam materials, including microcellular materials, can be processed by injecting a physical blowing agent into the polymeric material within the barrel. Many conventional blowing agent delivery systems inject blowing agents continuously into the polymeric material within the barrel. In discontinuous processes, such continuous delivery systems may prevent control over the percent blowing agent injected into the polymeric material and may lead to an uneven distribution of the blowing agent in the polymeric melt. In particular, the polymeric material in the vicinity of the blowing agent port, when the screw ceases to plasticate polymeric material, may contain higher amounts of blowing agent because of its increased residence time in proximity with the blowing agent injection port. The uneven distribution of blowing agent may result in viscosity variations within the polymeric material which can cause output inconsistencies in the extruder and other problems. Such effects reduce control over the process and narrow the processing window.
In some polymer processes including some discontinuous processes, such conventional blowing agent delivery systems may be adequate. However, in other processes such as discontinuous processes that require relatively precise control over blowing agent delivery, the conventional systems may reduce the effectiveness of the process. For example, certain processes for producing microcellular material may be adversely effected if the blowing agent is not precisely controlled.
Accordingly, a need exists for a system that delivers blowing agent in a controlled manner to a discontinuous polymer processing system.